Impact of Episodic Transport and Resuspension on Coastal Phytoplankton Processes:
A Case Study of the Lake Michigan Recurrent Coastal Plume

PIs: G. Fahnenstiel, S. Lohrenz, O. Schofield, D. Millie, L. Goad, and M. Julius

The purpose of this project was:

  1. characterize optical properties in the region of the RCP,
  2. determine the effects of the RCP on important phytoplankton rate process, (e.g. photosynthesis, growth, etc.), and
  3. determine the meroplanktonic potential of RCP species, with particular reference to the spring diatom bloom.

At this point in the project, all field experiments have been completed, and analysis and synthesis of results have begun. Much of the bulk and community level analyses are near completion, and several manuscripts are in preparation (a few even completed!). Taxon-specific information is still being counted, and likely will not be completed for another year. A brief summary of results is provided.

A large gradient in water-column light attenuation (KPAR) and concentrations of suspended particulate matter (SPM) was noted in southern Lake Michigan during the field program. Scattering was the dominant factor contributing to variability in light attenuation in the study area, especially at low wavelengths. Variability in particulate absorption was the primary source of variation in total absorption. Neither KPAR values nor SPM concentrations corresponded with chlorophyll (Chl) a concentrations, indicating no differences in phytoplankton biomass between plume- and non-impacted waters. Moreover, phytoplankton growth rates were also not correlated with SPM or KPAR , nor were significant differences noted between plume and non-plume stations. Sub-optimal maximum photosynthetic quantum yields were also indicative of constraints on phytoplankton growth throughout the study area.

Phytoplankton did not exhibit dramatic differences in the spectral shape of pigment absorption across plume gradients, and this was consistent with single cell determinations of absorption spectra. Differences in photoacclimation were apparently not occurring at a rate sufficient to produce clear spatial gradients in community pigment absorption. Diatoms and cryptophytes dominated the assemblages, often comprising greater than 85% of the Chl biomass. The positive associations of SPM concentrations and KPAR values with the relative contribution of diatoms to Chl a and KPAR values with the absolute contribution of diatoms to Chl a, along with the inverse relationship between the relative contributions of diatoms and cryptophytes to Chl a, corresponded with the spatial dominance of diatoms and cryptophytes in near- and offshore waters. The large, chain-forming centric diatoms that typically dominate Lake Michigan assemblages during the spring diatom bloom were not abundant within plume-impacted waters; rather, small centric diatoms were abundant.

The occurrence of fucoxanthin and Chl a, along with the abundance of small centric diatoms within the surficial sediment layer, established the presence of diatoms and confirmed this layer as a potential source of meroplankton. A large scale experiment conducted in the spring of 2000, verified the importance of meroplanktonic diatoms, and the limited role of plume materials in stimulating phytoplankton growth. Although resuspension of benthic diatoms appears to be important to the development of phytoplankton assemblages within nearshore, plume-impacted waters, it has little, if any relationship with the development of the annual, spring diatom bloom.